This invention relates in general to a monolithic silicon pressure sensor and more particularly to a single element pressure sensor which utilizes the shear stress piezoresistive effect in silicon.
The resistance value of a properly oriented silicon resistor is known to change in response to a flexing of the silicon crystal. This piezoresistive response has been utilized to fabricate pressure transducers. A differential pressure applied across a silicon diaphragm in which a resistor is formed causes a change in the resistor value. Measurements of the resistance change are proportional to changes in the differential pressure.
In the past it has not been found practical to use a single resistor as a pressure sensor. In general, the percentage change in resistance of the single resistor transducer is too low to be practical; the small signal that is generated requires sensitive amplifiers and engenders problems relating, for example, to noise. A more conventional approach has been, therefore, to arrange four resistors in a Wheatstone bridge configuration. Small changes in the individual resistance values contribute to a significant offset in the bridge and provide an easily detectible signal. A series of other problems, however, confront the Wheatstone bridge approach. The four resistors used for the bridge must be closely matched in value to avoid a zero offset; that is, a non-zero output for zero applied pressure differential. For diffused resistors this requires that both the geometry and diffusion parameters be controlled and uniform. More importantly the four resistors must have the same temperature coefficient; that is, the resistance value of each component must change with temperature in the same fashion. While it is possible, although undesirable, to correct for differences in component values which lead to a zero offset, it is very difficult to correct for differences in thermal coefficient. If the temperature coefficients of the four resistors are not identical each component will change in some different manner, changing the zero offset as the temperature changes.
In addition, most applications require that the pressure transducer have an output which is approximately linear with changes in pressure. The Wheatstone bridge generally has a relatively low output when designed for linear response, and low outputs are generally characterized by difficulties with noise and need for amplification.
Accordingly, in view of the disadvantages associated with Wheatstone bridge and other prior art pressure transducers, a need existed for an improved pressure transducer. It is therefore an object of this invention to provide a silicon pressure transducer which does not require matching of transducer components.
It is a further object of this invention to provide a silicon pressure transducer having improved reproducibility and improved temperature coefficient of response.
It is a still further object of this invention to provide a silicon pressure transducer having a small zero pressure differential offset.